Products such as hair spray and shaving cream that are sold in containers such as aerosol cans are usually opaque because the pressure required to facilitate the dispensing of the product must be contained in a strong container. The container must be strong enough to withstand substantial internal pressures that are used to force the product out of the container via an outlet that is opened and closed by means of a valve that is activated by the user of the product. Opaque metal cans are generally used instead of transparent materials such as glass to insure that the container can withstand the internal pressure and external handling. Unlike a transparent container you cannot see how much product is remaining in the can after you begin dispensing the product. It is desirable to know how much product remains in a can.